Except nonplussed isn't a fancy word with pedigree, it's something that you would assume originated in 1984 as one of the words in the simplified language for proles.
Not the GP, but imagine it has to do with "Newspeak" using the prefixes "plus" and "doubleplus" for emphasis. Nonplussed may have had a similar ring to "plusungood".
Actually, the proles in 1984 used normal English (as we read in the scene where Winston goes drinking in the proletarian sector and chats with the old man about life before Ingsoc). Newspeak was for the Party members.
